Question

Coefficient of emission of an opaque body is $0.6$. Radiant heat energy is incident on it at the rate of$20watts$. Determine the quantity of radiant heat reflected by it in $2$minutes​.
A. $480J$
B. $960J$
C. $1920J$
D. $1500J$

Answer 1

Hint: In order to solve this question, we need to find the emissivity of the body given. Emissivity is the measure of degree to be able to radiate energy by a body. Use this concept to find the answer to this question.

Formula used:
$E=\epsilon Pt$where $\epsilon =$emissivity, $P=$Power, $t=$time taken

Complete answer:
Emissivity, $\epsilon =1-0.6$ $\therefore$because it is an opaque body.
$\epsilon =0.4$
We have provided $P=$$20watts$, $t=2$minutes and $\epsilon =0.4$
Substituting all these value in below formula we get energy:
$$\begin{gathered} E=\epsilon Pt \\ E=0.4\times 20\times 2\times 60 \\ E=960J \end{gathered}$$(convert minutes into seconds by multiplying it by $60$)

The radiant heat energy is option(B) $960J$

Note: Points to be kept in mind while solving these questions:
Should have proper knowledge of emissivity because emissivity can be confusing sometimes.
Should learn all the important formulas as this topic contains lots of formulas which can be messy.